1. Field of the Invention
The present invention generally relates to nanoscale phosphor particles having a high internal quantum efficiency (IQE) useful for light emitting diodes (LED) and other optical devices.
2. Description of the Related Art
The particle size of phosphor materials utilized in applications such as fluorescent lamps, cathode ray tubes (CRTs), plasma display panels (PDPs), and white LEDs is usually around 1 μm to 10 μm. Additionally, smaller sized phosphor powder, e.g., less than 1 μm, have been used in PDPs. Recently, nanoscale phosphor materials have attracted a lot of attention due, in part, to their improved resolution in display devices and lowered scattering loss in the phosphor layer. These phosphor materials are usually manufactured using solid state reaction processes, which include a mixing process of raw material powders, sintering process under high temperature and high pressure, and pulverization of the sintered ceramic compacts to make micron sized phosphor powder. This process is also known as a “break down method,” which reflects upon the pulverization process that includes grinding of the particles into the smaller size.
However, using the aforementioned break down method tends to negatively affect the desired property of a high IQE of the phosphor particle materials. This is due, in part, to the generation of surface defects that occurs with the increased grinding of the particles to render them smaller. In addition to the surface defects caused by the grinding, there is no known practical method of physically grinding the phosphor powder down to a particle size of less than 200 nm using the break down methods.
Recently, nanoscale phosphor particle synthesis via “bottom up methods” has been used. For example, see U.S. Patent Application Publication No. 2006/0166057 to Kodas et al., U.S. Pat. No. 5,885,492 to Lee et al., U.S. Pat. No. 7,101,520 to Kumar, and U.S. Pat. No. 6,692,660 to Kumar, the contents of each reference is hereby incorporated by reference in its entirety. It is possible to synthesize nano-sized phosphor particles with relatively few surface defects when phosphor particles are built up from the atomic level in these so-called bottom up methods. However, surface atoms develop on the phosphor particles as the particle size decreases, resulting in an adverse drop in the IQE. For example, when the average particle size of the phosphor particles manufactured in the bottom up method become less than 200 nm, less than 100 nm, less than 50 nm, and less than 30 nm, the increased portion of surface atoms significantly lower the IQE. As reported in R. Kasuya et al., “Photoluminescence Enhancement of PEG-Modified YAG:Ce3+ Nanocrystal Phosphor Prepared by Glycothermal Method,” J. Phys. Chem. B 2005, 109, 22126-22130 and R. Kasuya et al., “Glycothermal synthesis and photoluminescence of YAG:Ce3+ nanophosphors,” Journal of Alloys and Compounds 408-412 (2006) 820-823 (the contents of both reference is hereby incorporated by reference in their entireties), surface passivation by organic compounds is critical, and they reported a phosphor particle IQE of 37.9% by properly surface treated nanoscale phosphor particle.
It is also reported that IQE tends to decrease about 1/7-⅛ of the initial IQE value when the organic compound for the surface passivation layer is removed by thermal decomposition at a temperature of around 1000 K. Therefore, it has been very difficult to obtain nanoscale phosphor particles with a reasonable IQE comparable to micron sized phosphor particle unless appropriate surface passivation is applied. The trade-off of manufacturing smaller particle sizes has come at a cost of lower IQE. Moreover, even after surface passivation, the reported highest value of IQE is still lower than 40%.